Spinal surgery using one or more surgical implants to stabilize, manipulate, and/or repair the spine is well known in the art. One type of common spinal surgery involves fusing or stabilizing two or more vertebra by application of a surgical construct to the posterior surfaces of the vertebra by means of pedicle screws.
There is a large market for pedicle screws and there are numerous designs and manufacturers of this type of fusion device. The spine market in the U.S. is $6.8 billion, and 34% of this market (over $2 billion) involves pedicle screw systems. These systems are usually placed bilaterally and the system on each side is typically composed of a minimum of one stabilizing rod, a pedicle screw for each vertebra, and a set screw at each pedicle screw to secure the stabilizing rod. Sometimes, the securing feature at the head of the pedicle screw is a separate connector. Each company has a slightly different design of the components, but generally, all pedicle screw constructs require a set screw to be tightened to a specific torque to ensure a proper connection between the pedicle screw and stabilizing rod, and thus a rigid fixation. It has been found that if the torque applied to the set screw is insufficient, the construct will loose integrity and the stabilizing rod will not be rigidly fixed as required and could slide or rotate. Additionally, an application of too much torque, it has been found, can result in a fracture of the vertebra or a loosening of the bone-implant connection. Too much torque can also severely deform the screw threads causing them to loose strength and to slip when the patient later puts a load on the spine or surgical construct.
Initially, a surgeon using these types of set screws would simply tighten them by hand until the surgeon judged that the proper tightness had been achieved. The problem with this approach was that there was no objective way for the surgeon to determine whether the set screw had been tightened to the required torque and the surgeon could easily apply too little or too much torque. And if there were a problem with the construct either during surgery or later, it was impossible for the surgeon to prove that the proper amount of torque had been applied.
To address these issues, a variety of systems were developed that utilized torque wrenches of various designs. These systems either required the surgeon to read the torque off the instrument during surgery or provided an audible sound and rotational slip when the proper torque had been reached. One problem with these prior art systems was the difficulty involved in reading the torque measurements or hearing and identifying the sound during surgery. In addition, the torque wrenches used in these systems could loose their precision with use and fail to undergo rotational slip at the target torque.
In another prior art system, the problems of the torque wrench based systems were avoided by means of breakaway set screws having a head designed to shear off the threaded body of the set screw once the proper torque has been achieved. While there are a variety of configurations known in the art, breakaway set screws are ordinarily made from a single piece of titanium alloy and have a hexagonal top portion that mates with a tightening device, a lower threaded set screw portion that mates with a threaded bore of a pedicle screw construct to secure a stabilizing rod, and an annular v-shaped notch separating the two portions.
In these prior art systems, the surgeon uses an extended counter torque tool that holds the top of the pedicle screw and stabilizing rod to try to limit or prevent transmission of the rotational torque used to tighten the set screw from being transmitted to the construct as a whole or to the vertebra of the patient. The shaft of the counter torque tool is hollow and sized to receive the shaft of a break off driver. The break off driver is longer than the counter torque tool and slides through the shaft of the counter torque tool to mate with the hexagonal head of the set screw. As set forth above, the hexagonal heads of these breakaway set screws are designed to shear off the threaded body of the set screw once the proper torque has been achieved. The surgeon simply turns the break off tool while keeping the counter torque tool still, until the hexagonal head shears off the threaded body of the set screw at the pre-determined torque. This set screw break off (“SSBO”) procedure is repeated for all of the set screws in the construct. The SSBO procedure is performed 6 times for the average spinal implant construct and many more times for larger constructs in patients with severe deformities such as scoliosis.
Unfortunately, each SSBO imparts an immense, if short lived, shock to both the patient and the surgeon due to the energy released during the catastrophic failure of the metal at the V-shaped notch when the hexagonal head separates from the lower threaded set screw portion. Bench top studies of a prior set screw using accelerometers at various points on and around the pedicle screw have recorded a shock of from about 200 g to about 800 g depending upon a variety of human factors, including how the tool was being held by the surgeon. (FIG. 1A-B) This shock creates significant problems for both the patient and the surgeon. It can lead to the pedicle screw breaking through the side of the vertebra or fracturing the vertebra. The shock can also reduce the pull out strength of the pedicle screw in the patient, thus increasing the chance of a later revision surgery being required. These risks are particularly high for patients suffering with osteoporosis. Further, the repeated shock may also cause premature wear and/or injury to the surgeon's hands and significantly increases the chance that the tools could slip in the surgeon's hands causing pain or injury to the patient.
Accordingly, there is a need in the art for a breakoff set screw for use within a spinal surgery construct, wherein the shock to the patient and physician from the SSBO is reduced.